.\" $OpenBSD: EVP_AEAD_CTX_init.3,v 1.16 2024/07/21 08:36:43 tb Exp $
.\"
.\" Copyright (c) 2014, Google Inc.
.\" Parts of the text were written by Adam Langley and David Benjamin.
.\" Copyright (c) 2015 Reyk Floeter <reyk@openbsd.org>
.\" Copyright (c) 2023 Ingo Schwarze <schwarze@openbsd.org>
.\"
.\" Permission to use, copy, modify, and/or distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd $Mdocdate: July 21 2024 $
.Dt EVP_AEAD_CTX_INIT 3
.Os
.Sh NAME
.Nm EVP_AEAD_CTX_new ,
.Nm EVP_AEAD_CTX_free ,
.Nm EVP_AEAD_CTX_init ,
.Nm EVP_AEAD_CTX_cleanup ,
.Nm EVP_AEAD_CTX_open ,
.Nm EVP_AEAD_CTX_seal ,
.Nm EVP_AEAD_key_length ,
.Nm EVP_AEAD_max_overhead ,
.Nm EVP_AEAD_max_tag_len ,
.Nm EVP_AEAD_nonce_length ,
.Nm EVP_aead_aes_128_gcm ,
.Nm EVP_aead_aes_256_gcm ,
.Nm EVP_aead_chacha20_poly1305 ,
.Nm EVP_aead_xchacha20_poly1305
.Nd authenticated encryption with additional data
.Sh SYNOPSIS
.In openssl/evp.h
.Ft EVP_AEAD_CTX *
.Fn EVP_AEAD_CTX_new void
.Ft void
.Fo EVP_AEAD_CTX_free
.Fa "EVP_AEAD_CTX *ctx"
.Fc
.Ft int
.Fo EVP_AEAD_CTX_init
.Fa "EVP_AEAD_CTX *ctx"
.Fa "const EVP_AEAD *aead"
.Fa "const unsigned char *key"
.Fa "size_t key_len"
.Fa "size_t tag_len"
.Fa "ENGINE *engine"
.Fc
.Ft void
.Fo EVP_AEAD_CTX_cleanup
.Fa "EVP_AEAD_CTX *ctx"
.Fc
.Ft int
.Fo EVP_AEAD_CTX_open
.Fa "const EVP_AEAD_CTX *ctx"
.Fa "unsigned char *out"
.Fa "size_t *out_len"
.Fa "size_t max_out_len"
.Fa "const unsigned char *nonce"
.Fa "size_t nonce_len"
.Fa "const unsigned char *in"
.Fa "size_t in_len"
.Fa "const unsigned char *ad"
.Fa "size_t ad_len"
.Fc
.Ft int
.Fo EVP_AEAD_CTX_seal
.Fa "const EVP_AEAD_CTX *ctx"
.Fa "unsigned char *out"
.Fa "size_t *out_len"
.Fa "size_t max_out_len"
.Fa "const unsigned char *nonce"
.Fa "size_t nonce_len"
.Fa "const unsigned char *in"
.Fa "size_t in_len"
.Fa "const unsigned char *ad"
.Fa "size_t ad_len"
.Fc
.Ft size_t
.Fo EVP_AEAD_key_length
.Fa "const EVP_AEAD *aead"
.Fc
.Ft size_t
.Fo EVP_AEAD_max_overhead
.Fa "const EVP_AEAD *aead"
.Fc
.Ft size_t
.Fo EVP_AEAD_max_tag_len
.Fa "const EVP_AEAD *aead"
.Fc
.Ft size_t
.Fo EVP_AEAD_nonce_length
.Fa "const EVP_AEAD *aead"
.Fc
.Ft const EVP_AEAD *
.Fo EVP_aead_aes_128_gcm
.Fa void
.Fc
.Ft const EVP_AEAD *
.Fo EVP_aead_aes_256_gcm
.Fa void
.Fc
.Ft const EVP_AEAD *
.Fo EVP_aead_chacha20_poly1305
.Fa void
.Fc
.Ft const EVP_AEAD *
.Fo EVP_aead_xchacha20_poly1305
.Fa void
.Fc
.Sh DESCRIPTION
AEAD (Authenticated Encryption with Additional Data) couples
confidentiality and integrity in a single primitive.
AEAD algorithms take a key and can then seal and open individual
messages.
Each message has a unique, per-message nonce and, optionally, additional
data which is authenticated but not included in the output.
.Pp
.Fn EVP_AEAD_CTX_new
allocates a new context for use with
.Fn EVP_AEAD_CTX_init .
It can be cleaned up for reuse with
.Fn EVP_AEAD_CTX_cleanup
and must be freed with
.Fn EVP_AEAD_CTX_free .
.Pp
.Fn EVP_AEAD_CTX_free
cleans up
.Fa ctx
and frees the space allocated to it.
.Pp
.Fn EVP_AEAD_CTX_init
initializes the context
.Fa ctx
for the given AEAD algorithm
.Fa aead .
The
.Fa engine
argument must be
.Dv NULL
for the default implementation;
other values are not supported.
Authentication tags may be truncated by passing a tag length.
A
.Fa tag_len
argument of
.Dv EVP_AEAD_DEFAULT_TAG_LENGTH ,
which has the value 0, causes the default tag length to be used.
.Pp
.Fn EVP_AEAD_CTX_cleanup
frees any data allocated for the context
.Fa ctx .
After
.Fn EVP_AEAD_CTX_cleanup ,
.Fa ctx
is in the same state as after
.Fn EVP_AEAD_CTX_new .
.Pp
.Fn EVP_AEAD_CTX_open
authenticates the input
.Fa in
and optional additional data
.Fa ad ,
decrypting the input and writing it as output
.Fa out .
This function may be called (with the same
.Vt EVP_AEAD_CTX )
concurrently with itself or with
.Fn EVP_AEAD_CTX_seal .
At most the number of input bytes are written as output.
In order to ensure success,
.Fa max_out_len
should be at least the same as the input length
.Fa in_len .
On successful return
.Fa out_len
is set to the actual number of bytes written.
The length of the
.Fa nonce
specified with
.Fa nonce_len
must be equal to the result of EVP_AEAD_nonce_length for this AEAD.
.Fn EVP_AEAD_CTX_open
never results in partial output.
If
.Fa max_out_len
is insufficient, zero will be returned and
.Fa out_len
will be set to zero.
If the input and output are aliased then
.Fa out
must be <=
.Fa in .
.Pp
.Fn EVP_AEAD_CTX_seal
encrypts and authenticates the input and authenticates any additional
data provided in
.Fa ad ,
the encrypted input and authentication tag being written as output
.Fa out .
This function may be called (with the same
.Vt EVP_AEAD_CTX )
concurrently with itself or with
.Fn EVP_AEAD_CTX_open .
At most
.Fa max_out_len
bytes are written as output and, in order to ensure success, this value
should be the
.Fa in_len
plus the result of
.Fn EVP_AEAD_max_overhead .
On successful return,
.Fa out_len
is set to the actual number of bytes written.
The length of the
.Fa nonce
specified with
.Fa nonce_len
must be equal to the result of
.Fn EVP_AEAD_nonce_length
for this AEAD.
.Fn EVP_AEAD_CTX_seal
never results in a partial output.
If
.Fa max_out_len
is insufficient, zero will be returned and
.Fa out_len
will be set to zero.
If the input and output are aliased then
.Fa out
must be <=
.Fa in .
.Pp
.Fn EVP_AEAD_key_length ,
.Fn EVP_AEAD_max_overhead ,
.Fn EVP_AEAD_max_tag_len ,
and
.Fn EVP_AEAD_nonce_length
provide information about the AEAD algorithm
.Fa aead .
.Pp
.Fn EVP_AEAD_max_tag_len
returns the maximum tag length that can be used with the given
.Fa aead .
This is the largest value that can be passed as the
.Fa tag_len
argument to
.Fn EVP_AEAD_CTX_init .
No built-in
.Vt EVP_AEAD
object has a maximum tag length larger than the constant
.Dv EVP_AEAD_MAX_TAG_LENGTH .
.Pp
All cipher algorithms have a fixed key length unless otherwise stated.
The following ciphers are available:
.Bl -tag -width Ds -offset indent
.It Fn EVP_aead_aes_128_gcm
AES-128 in Galois Counter Mode, using a
.Fa key_len
of 16 bytes and a
.Fa nonce_len
of 12 bytes.
.It Fn EVP_aead_aes_256_gcm
AES-256 in Galois Counter Mode, using a
.Fa key_len
of 32 bytes and a
.Fa nonce_len
of 12 bytes.
.It Fn EVP_aead_chacha20_poly1305
ChaCha20 with a Poly1305 authenticator, using a
.Fa key_len
of 32 bytes and a
.Fa nonce_len
of 12 bytes.
The constant
.Dv EVP_CHACHAPOLY_TLS_TAG_LEN
specifies the length of the authentication tag in bytes and has a value of 16.
.It Fn EVP_aead_xchacha20_poly1305
XChaCha20 with a Poly1305 authenticator, using a
.Fa key_len
of 32 bytes and a
.Fa nonce_len
of 24 bytes.
.El
.Pp
Unless compatibility with other implementations
like OpenSSL or BoringSSL is required, using the
.Sy EVP_AEAD
interface to AEAD ciphers is recommended
in preference to the functions documented in the
.Xr EVP_EncryptInit 3 ,
.Xr EVP_aes_256_gcm 3 ,
and
.Xr EVP_chacha20_poly1305 3
manual pages.
The code then becomes transparent to the AEAD cipher used
and much more flexible.
It is also safer to use as it prevents common mistakes with the EVP APIs.
.Sh RETURN VALUES
.Fn EVP_AEAD_CTX_new
returns the new
.Vt EVP_AEAD_CTX
object on success;
otherwise
.Dv NULL
is returned and
.Va errno
is set to
.Er ENOMEM .
.Pp
.Fn EVP_AEAD_CTX_init ,
.Fn EVP_AEAD_CTX_open ,
and
.Fn EVP_AEAD_CTX_seal
return 1 for success or zero for failure.
.Pp
.Fn EVP_AEAD_key_length
returns the length of the key used for this AEAD.
.Pp
.Fn EVP_AEAD_max_overhead
returns the maximum number of additional bytes added by the act of
sealing data with the AEAD.
.Pp
.Fn EVP_AEAD_max_tag_len
returns the maximum tag length when using this AEAD.
.Pp
.Fn EVP_AEAD_nonce_length
returns the length of the per-message nonce.
.Sh EXAMPLES
Encrypt a string using ChaCha20-Poly1305:
.Bd -literal -offset indent
const EVP_AEAD *aead = EVP_aead_chacha20_poly1305();
static const unsigned char nonce[32] = {0};
size_t buf_len, nonce_len;
EVP_AEAD_CTX *ctx;

ctx = EVP_AEAD_CTX_new();
EVP_AEAD_CTX_init(ctx, aead, key32, EVP_AEAD_key_length(aead),
    EVP_AEAD_DEFAULT_TAG_LENGTH, NULL);
nonce_len = EVP_AEAD_nonce_length(aead);

EVP_AEAD_CTX_seal(ctx, out, &out_len, BUFSIZE, nonce,
    nonce_len, in, in_len, NULL, 0);

EVP_AEAD_CTX_free(ctx);
.Ed
.Sh SEE ALSO
.Xr evp 3 ,
.Xr EVP_EncryptInit 3
.Sh STANDARDS
.Rs
.%A A. Langley
.%A W. Chang
.%A N. Mavrogiannopoulos
.%A J. Strombergson
.%A S. Josefsson
.%D June 2016
.%R RFC 7905
.%T ChaCha20-Poly1305 Cipher Suites for Transport Layer Security (TLS)
.Re
.Pp
.Rs
.%A S. Arciszewski
.%D October 2018
.%R draft-arciszewski-xchacha-02
.%T XChaCha: eXtended-nonce ChaCha and AEAD_XChaCha20_Poly1305
.Re
.Sh HISTORY
AEAD is based on the implementation by
.An Adam Langley
.\" OpenSSL commit 9a8646510b Sep 9 12:13:24 2013 -0400
for Chromium/BoringSSL and first appeared in
.Ox 5.6 .
.Pp
.Fn EVP_AEAD_CTX_new
and
.Fn EVP_AEAD_CTX_free
first appeared in
.Ox 7.1 .
.Sh CAVEATS
The original publications and code by
.An Adam Langley
used a modified AEAD construction that is incompatible with the common
style used by AEAD in TLS and incompatible with RFC 7905:
.Pp
.Rs
.%A A. Langley
.%A W. Chang
.%D November 2013
.%R draft-agl-tls-chacha20poly1305-04
.%T ChaCha20 and Poly1305 based Cipher Suites for TLS
.Re
.Pp
.Rs
.%A Y. Nir
.%A A. Langley
.%D June 2018
.%R RFC 8439
.%T ChaCha20 and Poly1305 for IETF Protocols
.Re
.Pp
In particular, the original version used a
.Fa nonce_len
of 8 bytes.
